RAID schemes have provided varying levels of redundancy for digital data on storage media for decades. Traditional and common RAID schemes all require additional capacity beyond the primary data capacity in order to maintain an online recoverable copy of the data in the event of primary source media failure. This benefit of protection comes at a cost. RAID schemes typically also represent additional performance overhead, as the data must be written to multiple times instead of just once. In addition to the increased write workload, latency may be increased due to the need to wait for these multiple writes to be completed and acknowledged before data commit and protection is assured.
Modern storage arrays utilize cache to mitigate this “write penalty,” but performance is impacted when cache is heavily utilized and write destaging needs to occur. Additionally, total write cache is commonly shared for all writes—both primary target and RAID protection—so that cache size must be increased in sizing and design to compensate for RAID utilization to avoid the impact of premature write destaging on production IO. Improper stripe alignment can aggravate RAID overhead and significantly increase IO as well. RAID schemes represent some amount of compromise between protection, usable capacity, and performance. These compromises commonly result in significant increased cost outlay when RAID is utilized for increased data protection. This cost comes from the need for additional capacity for parity or mirroring, as opposed to merely using RAID 0, which does not provide an increase in protection.
The RAID scheme chosen may determine the amount of protection and performance provided, with different schemes having varying characteristics around: amount of additional raw storage capacity required and resulting reduction of the usable percentage of overall storage, write penalties impacting performance, performance during a media failure, ability to withstand multiple media failures, time required to rebuild RAID protection after media failure, and performance impact of rebuilding RAID protection. These characteristics may not be flexibly adjusted to change each individual characteristic separately as desired, as there are dependencies based on the source physical (or virtual) configuration and limitations of each RAID scheme itself.
While certain configurations, such as Isilon storage from EMC Corporation of Hopkinton, Mass., use software protection rather than hardware-based RAID to provide as much as 80% guaranteed usable space, it is common in large storage environments for usable storage to be ˜66% of raw storage at best, with many storage architectures resulting in significantly worse usable to raw ratios.
Usable capacity reduction is not simply a result of RAID schemes, though RAID is commonly the primary factor. The method for calculating storage capacity itself (Base-2 conversion) is a factor, as are storage architecture limitations, disk “right-sizing” or simply architectural or environment standards requiring specific sizes for allocation that result in unutilized capacity. Some storage architectures also require “hold back” capacity or present some additional overhead, such as “snapshot reserve” or file system (if utilized) overhead. Additionally, hot spares are commonly provisioned and further reduce usable capacity.